Density Functional Theory Study On The Mechanism Of Heterogeneous Catalytic Reactions

It is of great significance to study the active site and reaction mechanism in heterogeneous catalysis.Due to the complexity of heterogeneous catalyst structures and the limitations of existing characterization technologies,the current understanding of the active sites and reaction mechanisms of catalysts still remain controversial.With the continuous development of computational chemistry,the study of catalyst structure and reaction mechanism based on density functional theory has been extensively used in heterogeneous catalytic reactions.At present,there are two difficulties in theoretical catalysis research:?1?How to obtain the mimimum reaction path in the global reaction potential energy surface??2?How to theoretically design the controllable synthesis of model catalysts with specific morphology?Aiming at the two difficult points in theoretical catalysis,this thesis systematically calculates the transition states of typical organic molecular reactions,continuous hydrogenation of C₂H₂ on the metal surface,and CO oxidation on the surface of metal oxides using LASP software.By adjusting the coverage of the adsorbed species on gold surfaces,the surface energies of the gold nanoparticles are changed with coverage,and then a model catalyst with a specific morphology is theoretically designed.The mechanism of C₂H₂ hydrogenation at the step and terrace sites on Au?997?surface was studied by using the double-ended TS search method in LASP package.The calculation results show that the adsorption of species at the step site is much higher than that at the terrace site.For the hydrogenation of C₂H₂ to C₂H₆ on Au?997?surface,the hydrogenation barrier of C₂H₂ is lower,while the barrier of C₂H₄ is the higher.This indicats that the hydrogen reaction is difficult on terrace,and C₂H₂ can be selectively hydrogenated to C₂H₄ on the step surface.The activation energy of the hydrogenation reaction is higher than that of the reverse reaction,indicating that the hydrogenation of acetylene on Au?997?is a thermodynamic exothermic process.The global searching method of potential energy surface by random walking on the surface was used to study the active sites and reaction mechanism of CO oxidation on NiO.The calculation results show that low coordination Ni ions are the active sites for CO oxidation.Lattice oxygen and adsorbed oxygen have lower CO oxidation activity;NiO_1-x?100?cannot maintain the CO oxidation cycle due to the higher reaction energy barrier for the regeneration of oxygen vacancies.The oxygen species on the step-NiO?100?has a lower potential barrier and maintains the catalytic cycle.It has shown that the preparation of NiO catalysts with many defects and low coordination numbers is the direction for designing and developing high-performance NiO-based catalysts.There are many transition state searching methods,such as CINEB,DIMER,DESW,LST/QST,etc.In this thesis,the hydrogenation of acetylene to ethylene on Au?997?surface was used as a probe reaction.The pontential efficient transition state searching method is suggested by considering the accuracy and efficiency.The results show that the structures of transition states obtained by CINEB,DIMER,DESW,LST/QST methonds are all reasonable.There is one imaginary frequency of transition state structure by DESW in LASP software;while virtual frequencies with low value appear by CINEB method.Furthermore,the accuracy and efficiency of transition state searching by DIMER is more dependent on the initially guessed configuration.It is also found that the calculation time of the CINEB method increases with the convergence criterion;while the calculation time of the DIMER is less than that of CINEB.Therefore,DESW might be suitable for the global search of transition states,while CINEB method can be used to find a reasonable initial guess configuration,and then precise transition state calculations can be performed by DIMER method.Theoretical design and controllable synthesis of catalysts with specific morphology or exposed specific crystal planes are the challenge in catalytsis science.In this thesis,gold nanoparticles were selected and the adsorption structure of CO molecules on different crystal surfaces of gold was calculated by density functional theory?DFT?.It was found that CO selectively adsorbs on different crystal surfaces of Au.This provides a theoretical basis to regulate the morphology of metal nanoparticles by using the selective adsorption of small molecules.By adjusting the adsorption coverages of CO on different surfaces,the surface energies of Au?111?,Au?110?and Au?100?at different temperatures and pressures,the relationship between the surface energy and the exposed surface of the metal nanoparticles was established.This provides a theoretical guidance for controllable synthesis of the catalyst.